x86, asm: Merge cmpxchg_486_u64() and cmpxchg8b_emu()

We have two functions for doing exactly the same thing -- emulating
cmpxchg8b on 486 and older hardware -- with different calling
conventions, and yet doing the same thing.  Drop the C version and use
the assembly version, via alternatives, for both the local and
non-local versions of cmpxchg8b.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
LKML-Reference: <AANLkTikAmaDPji-TVDarmG1yD=fwbffcsmEU=YEuP+8r@mail.gmail.com>